Gimme swelter: we explain the UK and US heatwaves

AFTER the cold, the heat. High pressure from Siberia last spring brought record cold temperatures to the UK. Now, more high pressure, this time from the tropical Atlantic, is behind the heatwave that caused grassfires in south-east England.

These high-pressure zones block the jet stream, which usually brings the country's changeable weather. "Blocking highs" are an increasing part of North American weather, too.

It is too soon to tell whether the events are part of a trend. Jennifer Francis at Rutgers University, New Jersey, reported last year that the jet stream, driven by the temperature difference between the Arctic and lower latitudes, appears to have slowed and is meandering more, perhaps because of the warming Arctic. This creates perfect conditions for blocking highs, she says.

But climate models tell a different story. Most predict a decrease in blocking highs, at least over Europe.

This article appeared in print under the headline "Blocked jet stream behind heat"

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